//
//===----------------------------------------------------------------------===//
-#define DEBUG_TYPE "machine-sink"
#include "llvm/CodeGen/Passes.h"
-#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
-#include "llvm/Analysis/AliasAnalysis.h"
-#include "llvm/Target/TargetRegisterInfo.h"
-#include "llvm/Target/TargetInstrInfo.h"
-#include "llvm/Target/TargetMachine.h"
-#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachinePostDominators.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
+#include "llvm/Target/TargetInstrInfo.h"
+#include "llvm/Target/TargetRegisterInfo.h"
+#include "llvm/Target/TargetSubtargetInfo.h"
using namespace llvm;
-static cl::opt<bool>
+#define DEBUG_TYPE "machine-sink"
+
+static cl::opt<bool>
SplitEdges("machine-sink-split",
cl::desc("Split critical edges during machine sinking"),
- cl::init(false), cl::Hidden);
-static cl::opt<unsigned>
-SplitLimit("split-limit",
- cl::init(~0u), cl::Hidden);
+ cl::init(true), cl::Hidden);
+
+static cl::opt<bool>
+UseBlockFreqInfo("machine-sink-bfi",
+ cl::desc("Use block frequency info to find successors to sink"),
+ cl::init(true), cl::Hidden);
+
-STATISTIC(NumSunk, "Number of machine instructions sunk");
-STATISTIC(NumSplit, "Number of critical edges split");
+STATISTIC(NumSunk, "Number of machine instructions sunk");
+STATISTIC(NumSplit, "Number of critical edges split");
+STATISTIC(NumCoalesces, "Number of copies coalesced");
namespace {
class MachineSinking : public MachineFunctionPass {
const TargetInstrInfo *TII;
const TargetRegisterInfo *TRI;
- MachineRegisterInfo *RegInfo; // Machine register information
- MachineDominatorTree *DT; // Machine dominator tree
+ MachineRegisterInfo *MRI; // Machine register information
+ MachineDominatorTree *DT; // Machine dominator tree
+ MachinePostDominatorTree *PDT; // Machine post dominator tree
MachineLoopInfo *LI;
+ const MachineBlockFrequencyInfo *MBFI;
AliasAnalysis *AA;
- BitVector AllocatableSet; // Which physregs are allocatable?
+
+ // Remember which edges have been considered for breaking.
+ SmallSet<std::pair<MachineBasicBlock*,MachineBasicBlock*>, 8>
+ CEBCandidates;
+ // Remember which edges we are about to split.
+ // This is different from CEBCandidates since those edges
+ // will be split.
+ SetVector<std::pair<MachineBasicBlock*,MachineBasicBlock*> > ToSplit;
public:
static char ID; // Pass identification
- MachineSinking() : MachineFunctionPass(ID) {}
+ MachineSinking() : MachineFunctionPass(ID) {
+ initializeMachineSinkingPass(*PassRegistry::getPassRegistry());
+ }
- virtual bool runOnMachineFunction(MachineFunction &MF);
+ bool runOnMachineFunction(MachineFunction &MF) override;
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.setPreservesCFG();
MachineFunctionPass::getAnalysisUsage(AU);
AU.addRequired<AliasAnalysis>();
AU.addRequired<MachineDominatorTree>();
+ AU.addRequired<MachinePostDominatorTree>();
AU.addRequired<MachineLoopInfo>();
AU.addPreserved<MachineDominatorTree>();
+ AU.addPreserved<MachinePostDominatorTree>();
AU.addPreserved<MachineLoopInfo>();
+ if (UseBlockFreqInfo)
+ AU.addRequired<MachineBlockFrequencyInfo>();
}
+
+ void releaseMemory() override {
+ CEBCandidates.clear();
+ }
+
private:
bool ProcessBlock(MachineBasicBlock &MBB);
- MachineBasicBlock *SplitCriticalEdge(MachineBasicBlock *From,
- MachineBasicBlock *To);
+ bool isWorthBreakingCriticalEdge(MachineInstr *MI,
+ MachineBasicBlock *From,
+ MachineBasicBlock *To);
+ /// \brief Postpone the splitting of the given critical
+ /// edge (\p From, \p To).
+ ///
+ /// We do not split the edges on the fly. Indeed, this invalidates
+ /// the dominance information and thus triggers a lot of updates
+ /// of that information underneath.
+ /// Instead, we postpone all the splits after each iteration of
+ /// the main loop. That way, the information is at least valid
+ /// for the lifetime of an iteration.
+ ///
+ /// \return True if the edge is marked as toSplit, false otherwise.
+ /// False can be returned if, for instance, this is not profitable.
+ bool PostponeSplitCriticalEdge(MachineInstr *MI,
+ MachineBasicBlock *From,
+ MachineBasicBlock *To,
+ bool BreakPHIEdge);
bool SinkInstruction(MachineInstr *MI, bool &SawStore);
bool AllUsesDominatedByBlock(unsigned Reg, MachineBasicBlock *MBB,
- MachineBasicBlock *DefMBB, bool &LocalUse) const;
+ MachineBasicBlock *DefMBB,
+ bool &BreakPHIEdge, bool &LocalUse) const;
+ MachineBasicBlock *FindSuccToSinkTo(MachineInstr *MI, MachineBasicBlock *MBB,
+ bool &BreakPHIEdge);
+ bool isProfitableToSinkTo(unsigned Reg, MachineInstr *MI,
+ MachineBasicBlock *MBB,
+ MachineBasicBlock *SuccToSinkTo);
+
+ bool PerformTrivialForwardCoalescing(MachineInstr *MI,
+ MachineBasicBlock *MBB);
};
} // end anonymous namespace
char MachineSinking::ID = 0;
-INITIALIZE_PASS(MachineSinking, "machine-sink",
- "Machine code sinking", false, false);
+char &llvm::MachineSinkingID = MachineSinking::ID;
+INITIALIZE_PASS_BEGIN(MachineSinking, "machine-sink",
+ "Machine code sinking", false, false)
+INITIALIZE_PASS_DEPENDENCY(MachineDominatorTree)
+INITIALIZE_PASS_DEPENDENCY(MachineLoopInfo)
+INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
+INITIALIZE_PASS_END(MachineSinking, "machine-sink",
+ "Machine code sinking", false, false)
+
+bool MachineSinking::PerformTrivialForwardCoalescing(MachineInstr *MI,
+ MachineBasicBlock *MBB) {
+ if (!MI->isCopy())
+ return false;
+
+ unsigned SrcReg = MI->getOperand(1).getReg();
+ unsigned DstReg = MI->getOperand(0).getReg();
+ if (!TargetRegisterInfo::isVirtualRegister(SrcReg) ||
+ !TargetRegisterInfo::isVirtualRegister(DstReg) ||
+ !MRI->hasOneNonDBGUse(SrcReg))
+ return false;
+
+ const TargetRegisterClass *SRC = MRI->getRegClass(SrcReg);
+ const TargetRegisterClass *DRC = MRI->getRegClass(DstReg);
+ if (SRC != DRC)
+ return false;
-FunctionPass *llvm::createMachineSinkingPass() { return new MachineSinking(); }
+ MachineInstr *DefMI = MRI->getVRegDef(SrcReg);
+ if (DefMI->isCopyLike())
+ return false;
+ DEBUG(dbgs() << "Coalescing: " << *DefMI);
+ DEBUG(dbgs() << "*** to: " << *MI);
+ MRI->replaceRegWith(DstReg, SrcReg);
+ MI->eraseFromParent();
+
+ // Conservatively, clear any kill flags, since it's possible that they are no
+ // longer correct.
+ MRI->clearKillFlags(SrcReg);
+
+ ++NumCoalesces;
+ return true;
+}
/// AllUsesDominatedByBlock - Return true if all uses of the specified register
/// occur in blocks dominated by the specified block. If any use is in the
/// definition block, then return false since it is never legal to move def
/// after uses.
-bool MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
- MachineBasicBlock *MBB,
- MachineBasicBlock *DefMBB,
- bool &LocalUse) const {
+bool
+MachineSinking::AllUsesDominatedByBlock(unsigned Reg,
+ MachineBasicBlock *MBB,
+ MachineBasicBlock *DefMBB,
+ bool &BreakPHIEdge,
+ bool &LocalUse) const {
assert(TargetRegisterInfo::isVirtualRegister(Reg) &&
"Only makes sense for vregs");
- // Ignoring debug uses is necessary so debug info doesn't affect the code.
- // This may leave a referencing dbg_value in the original block, before
- // the definition of the vreg. Dwarf generator handles this although the
- // user might not get the right info at runtime.
- for (MachineRegisterInfo::use_nodbg_iterator
- I = RegInfo->use_nodbg_begin(Reg), E = RegInfo->use_nodbg_end();
- I != E; ++I) {
- // Determine the block of the use.
- MachineInstr *UseInst = &*I;
+
+ // Ignore debug uses because debug info doesn't affect the code.
+ if (MRI->use_nodbg_empty(Reg))
+ return true;
+
+ // BreakPHIEdge is true if all the uses are in the successor MBB being sunken
+ // into and they are all PHI nodes. In this case, machine-sink must break
+ // the critical edge first. e.g.
+ //
+ // BB#1: derived from LLVM BB %bb4.preheader
+ // Predecessors according to CFG: BB#0
+ // ...
+ // %reg16385<def> = DEC64_32r %reg16437, %EFLAGS<imp-def,dead>
+ // ...
+ // JE_4 <BB#37>, %EFLAGS<imp-use>
+ // Successors according to CFG: BB#37 BB#2
+ //
+ // BB#2: derived from LLVM BB %bb.nph
+ // Predecessors according to CFG: BB#0 BB#1
+ // %reg16386<def> = PHI %reg16434, <BB#0>, %reg16385, <BB#1>
+ BreakPHIEdge = true;
+ for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
+ MachineInstr *UseInst = MO.getParent();
+ unsigned OpNo = &MO - &UseInst->getOperand(0);
MachineBasicBlock *UseBlock = UseInst->getParent();
+ if (!(UseBlock == MBB && UseInst->isPHI() &&
+ UseInst->getOperand(OpNo+1).getMBB() == DefMBB)) {
+ BreakPHIEdge = false;
+ break;
+ }
+ }
+ if (BreakPHIEdge)
+ return true;
+ for (MachineOperand &MO : MRI->use_nodbg_operands(Reg)) {
+ // Determine the block of the use.
+ MachineInstr *UseInst = MO.getParent();
+ unsigned OpNo = &MO - &UseInst->getOperand(0);
+ MachineBasicBlock *UseBlock = UseInst->getParent();
if (UseInst->isPHI()) {
// PHI nodes use the operand in the predecessor block, not the block with
// the PHI.
- UseBlock = UseInst->getOperand(I.getOperandNo()+1).getMBB();
+ UseBlock = UseInst->getOperand(OpNo+1).getMBB();
} else if (UseBlock == DefMBB) {
LocalUse = true;
return false;
}
bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
+ if (skipOptnoneFunction(*MF.getFunction()))
+ return false;
+
DEBUG(dbgs() << "******** Machine Sinking ********\n");
- const TargetMachine &TM = MF.getTarget();
- TII = TM.getInstrInfo();
- TRI = TM.getRegisterInfo();
- RegInfo = &MF.getRegInfo();
+ TII = MF.getSubtarget().getInstrInfo();
+ TRI = MF.getSubtarget().getRegisterInfo();
+ MRI = &MF.getRegInfo();
DT = &getAnalysis<MachineDominatorTree>();
+ PDT = &getAnalysis<MachinePostDominatorTree>();
LI = &getAnalysis<MachineLoopInfo>();
+ MBFI = UseBlockFreqInfo ? &getAnalysis<MachineBlockFrequencyInfo>() : nullptr;
AA = &getAnalysis<AliasAnalysis>();
- AllocatableSet = TRI->getAllocatableSet(MF);
bool EverMadeChange = false;
bool MadeChange = false;
// Process all basic blocks.
+ CEBCandidates.clear();
+ ToSplit.clear();
for (MachineFunction::iterator I = MF.begin(), E = MF.end();
I != E; ++I)
MadeChange |= ProcessBlock(*I);
+ // If we have anything we marked as toSplit, split it now.
+ for (auto &Pair : ToSplit) {
+ auto NewSucc = Pair.first->SplitCriticalEdge(Pair.second, this);
+ if (NewSucc != nullptr) {
+ DEBUG(dbgs() << " *** Splitting critical edge:"
+ " BB#" << Pair.first->getNumber()
+ << " -- BB#" << NewSucc->getNumber()
+ << " -- BB#" << Pair.second->getNumber() << '\n');
+ MadeChange = true;
+ ++NumSplit;
+ } else
+ DEBUG(dbgs() << " *** Not legal to break critical edge\n");
+ }
// If this iteration over the code changed anything, keep iterating.
if (!MadeChange) break;
EverMadeChange = true;
if (MI->isDebugValue())
continue;
+ bool Joined = PerformTrivialForwardCoalescing(MI, &MBB);
+ if (Joined) {
+ MadeChange = true;
+ continue;
+ }
+
if (SinkInstruction(MI, SawStore))
++NumSunk, MadeChange = true;
return MadeChange;
}
-MachineBasicBlock *MachineSinking::SplitCriticalEdge(MachineBasicBlock *FromBB,
- MachineBasicBlock *ToBB) {
+bool MachineSinking::isWorthBreakingCriticalEdge(MachineInstr *MI,
+ MachineBasicBlock *From,
+ MachineBasicBlock *To) {
+ // FIXME: Need much better heuristics.
+
+ // If the pass has already considered breaking this edge (during this pass
+ // through the function), then let's go ahead and break it. This means
+ // sinking multiple "cheap" instructions into the same block.
+ if (!CEBCandidates.insert(std::make_pair(From, To)).second)
+ return true;
+
+ if (!MI->isCopy() && !TII->isAsCheapAsAMove(MI))
+ return true;
+
+ // MI is cheap, we probably don't want to break the critical edge for it.
+ // However, if this would allow some definitions of its source operands
+ // to be sunk then it's probably worth it.
+ for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
+ const MachineOperand &MO = MI->getOperand(i);
+ if (!MO.isReg() || !MO.isUse())
+ continue;
+ unsigned Reg = MO.getReg();
+ if (Reg == 0)
+ continue;
+
+ // We don't move live definitions of physical registers,
+ // so sinking their uses won't enable any opportunities.
+ if (TargetRegisterInfo::isPhysicalRegister(Reg))
+ continue;
+
+ // If this instruction is the only user of a virtual register,
+ // check if breaking the edge will enable sinking
+ // both this instruction and the defining instruction.
+ if (MRI->hasOneNonDBGUse(Reg)) {
+ // If the definition resides in same MBB,
+ // claim it's likely we can sink these together.
+ // If definition resides elsewhere, we aren't
+ // blocking it from being sunk so don't break the edge.
+ MachineInstr *DefMI = MRI->getVRegDef(Reg);
+ if (DefMI->getParent() == MI->getParent())
+ return true;
+ }
+ }
+
+ return false;
+}
+
+bool MachineSinking::PostponeSplitCriticalEdge(MachineInstr *MI,
+ MachineBasicBlock *FromBB,
+ MachineBasicBlock *ToBB,
+ bool BreakPHIEdge) {
+ if (!isWorthBreakingCriticalEdge(MI, FromBB, ToBB))
+ return false;
+
// Avoid breaking back edge. From == To means backedge for single BB loop.
- if (!SplitEdges || NumSplit == SplitLimit || FromBB == ToBB)
- return 0;
-
- // Check for more "complex" loops.
- if (LI->getLoopFor(FromBB) != LI->getLoopFor(ToBB) ||
- !LI->isLoopHeader(ToBB)) {
- // It's not always legal to break critical edges and sink the computation
- // to the edge.
- //
- // BB#1:
- // v1024
- // Beq BB#3
- // <fallthrough>
- // BB#2:
- // ... no uses of v1024
- // <fallthrough>
- // BB#3:
- // ...
- // = v1024
- //
- // If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted:
- //
- // BB#1:
- // ...
- // Bne BB#2
- // BB#4:
- // v1024 =
- // B BB#3
- // BB#2:
- // ... no uses of v1024
- // <fallthrough>
- // BB#3:
- // ...
- // = v1024
- //
- // This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3
- // flow. We need to ensure the new basic block where the computation is
- // sunk to dominates all the uses.
- // It's only legal to break critical edge and sink the computation to the
- // new block if all the predecessors of "To", except for "From", are
- // not dominated by "From". Given SSA property, this means these
- // predecessors are dominated by "To".
+ if (!SplitEdges || FromBB == ToBB)
+ return false;
+
+ // Check for backedges of more "complex" loops.
+ if (LI->getLoopFor(FromBB) == LI->getLoopFor(ToBB) &&
+ LI->isLoopHeader(ToBB))
+ return false;
+
+ // It's not always legal to break critical edges and sink the computation
+ // to the edge.
+ //
+ // BB#1:
+ // v1024
+ // Beq BB#3
+ // <fallthrough>
+ // BB#2:
+ // ... no uses of v1024
+ // <fallthrough>
+ // BB#3:
+ // ...
+ // = v1024
+ //
+ // If BB#1 -> BB#3 edge is broken and computation of v1024 is inserted:
+ //
+ // BB#1:
+ // ...
+ // Bne BB#2
+ // BB#4:
+ // v1024 =
+ // B BB#3
+ // BB#2:
+ // ... no uses of v1024
+ // <fallthrough>
+ // BB#3:
+ // ...
+ // = v1024
+ //
+ // This is incorrect since v1024 is not computed along the BB#1->BB#2->BB#3
+ // flow. We need to ensure the new basic block where the computation is
+ // sunk to dominates all the uses.
+ // It's only legal to break critical edge and sink the computation to the
+ // new block if all the predecessors of "To", except for "From", are
+ // not dominated by "From". Given SSA property, this means these
+ // predecessors are dominated by "To".
+ //
+ // There is no need to do this check if all the uses are PHI nodes. PHI
+ // sources are only defined on the specific predecessor edges.
+ if (!BreakPHIEdge) {
for (MachineBasicBlock::pred_iterator PI = ToBB->pred_begin(),
E = ToBB->pred_end(); PI != E; ++PI) {
if (*PI == FromBB)
continue;
if (!DT->dominates(ToBB, *PI))
- return 0;
+ return false;
}
-
- // FIXME: Determine if it's cost effective to break this edge.
- return FromBB->SplitCriticalEdge(ToBB, this);
}
- return 0;
+ ToSplit.insert(std::make_pair(FromBB, ToBB));
+
+ return true;
}
-/// SinkInstruction - Determine whether it is safe to sink the specified machine
-/// instruction out of its current block into a successor.
-bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
- // Check if it's safe to move the instruction.
- if (!MI->isSafeToMove(TII, AA, SawStore))
+static bool AvoidsSinking(MachineInstr *MI, MachineRegisterInfo *MRI) {
+ return MI->isInsertSubreg() || MI->isSubregToReg() || MI->isRegSequence();
+}
+
+/// collectDebgValues - Scan instructions following MI and collect any
+/// matching DBG_VALUEs.
+static void collectDebugValues(MachineInstr *MI,
+ SmallVectorImpl<MachineInstr *> &DbgValues) {
+ DbgValues.clear();
+ if (!MI->getOperand(0).isReg())
+ return;
+
+ MachineBasicBlock::iterator DI = MI; ++DI;
+ for (MachineBasicBlock::iterator DE = MI->getParent()->end();
+ DI != DE; ++DI) {
+ if (!DI->isDebugValue())
+ return;
+ if (DI->getOperand(0).isReg() &&
+ DI->getOperand(0).getReg() == MI->getOperand(0).getReg())
+ DbgValues.push_back(DI);
+ }
+}
+
+/// isProfitableToSinkTo - Return true if it is profitable to sink MI.
+bool MachineSinking::isProfitableToSinkTo(unsigned Reg, MachineInstr *MI,
+ MachineBasicBlock *MBB,
+ MachineBasicBlock *SuccToSinkTo) {
+ assert (MI && "Invalid MachineInstr!");
+ assert (SuccToSinkTo && "Invalid SinkTo Candidate BB");
+
+ if (MBB == SuccToSinkTo)
return false;
- // FIXME: This should include support for sinking instructions within the
- // block they are currently in to shorten the live ranges. We often get
- // instructions sunk into the top of a large block, but it would be better to
- // also sink them down before their first use in the block. This xform has to
- // be careful not to *increase* register pressure though, e.g. sinking
- // "x = y + z" down if it kills y and z would increase the live ranges of y
- // and z and only shrink the live range of x.
+ // It is profitable if SuccToSinkTo does not post dominate current block.
+ if (!PDT->dominates(SuccToSinkTo, MBB))
+ return true;
+
+ // It is profitable to sink an instruction from a deeper loop to a shallower
+ // loop, even if the latter post-dominates the former (PR21115).
+ if (LI->getLoopDepth(MBB) > LI->getLoopDepth(SuccToSinkTo))
+ return true;
+
+ // Check if only use in post dominated block is PHI instruction.
+ bool NonPHIUse = false;
+ for (MachineInstr &UseInst : MRI->use_nodbg_instructions(Reg)) {
+ MachineBasicBlock *UseBlock = UseInst.getParent();
+ if (UseBlock == SuccToSinkTo && !UseInst.isPHI())
+ NonPHIUse = true;
+ }
+ if (!NonPHIUse)
+ return true;
+
+ // If SuccToSinkTo post dominates then also it may be profitable if MI
+ // can further profitably sinked into another block in next round.
+ bool BreakPHIEdge = false;
+ // FIXME - If finding successor is compile time expensive then cache results.
+ if (MachineBasicBlock *MBB2 = FindSuccToSinkTo(MI, SuccToSinkTo, BreakPHIEdge))
+ return isProfitableToSinkTo(Reg, MI, SuccToSinkTo, MBB2);
+
+ // If SuccToSinkTo is final destination and it is a post dominator of current
+ // block then it is not profitable to sink MI into SuccToSinkTo block.
+ return false;
+}
+
+/// FindSuccToSinkTo - Find a successor to sink this instruction to.
+MachineBasicBlock *MachineSinking::FindSuccToSinkTo(MachineInstr *MI,
+ MachineBasicBlock *MBB,
+ bool &BreakPHIEdge) {
+
+ assert (MI && "Invalid MachineInstr!");
+ assert (MBB && "Invalid MachineBasicBlock!");
// Loop over all the operands of the specified instruction. If there is
// anything we can't handle, bail out.
- MachineBasicBlock *ParentBlock = MI->getParent();
// SuccToSinkTo - This is the successor to sink this instruction to, once we
// decide.
- MachineBasicBlock *SuccToSinkTo = 0;
-
+ MachineBasicBlock *SuccToSinkTo = nullptr;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isReg()) continue; // Ignore non-register operands.
// If the physreg has no defs anywhere, it's just an ambient register
// and we can freely move its uses. Alternatively, if it's allocatable,
// it could get allocated to something with a def during allocation.
- if (!RegInfo->def_empty(Reg))
- return false;
-
- if (AllocatableSet.test(Reg))
- return false;
-
- // Check for a def among the register's aliases too.
- for (const unsigned *Alias = TRI->getAliasSet(Reg); *Alias; ++Alias) {
- unsigned AliasReg = *Alias;
- if (!RegInfo->def_empty(AliasReg))
- return false;
-
- if (AllocatableSet.test(AliasReg))
- return false;
- }
+ if (!MRI->isConstantPhysReg(Reg, *MBB->getParent()))
+ return nullptr;
} else if (!MO.isDead()) {
// A def that isn't dead. We can't move it.
- return false;
+ return nullptr;
}
} else {
// Virtual register uses are always safe to sink.
if (MO.isUse()) continue;
// If it's not safe to move defs of the register class, then abort.
- if (!TII->isSafeToMoveRegClassDefs(RegInfo->getRegClass(Reg)))
- return false;
-
- // FIXME: This picks a successor to sink into based on having one
- // successor that dominates all the uses. However, there are cases where
- // sinking can happen but where the sink point isn't a successor. For
- // example:
- //
- // x = computation
- // if () {} else {}
- // use x
- //
- // the instruction could be sunk over the whole diamond for the
- // if/then/else (or loop, etc), allowing it to be sunk into other blocks
- // after that.
+ if (!TII->isSafeToMoveRegClassDefs(MRI->getRegClass(Reg)))
+ return nullptr;
// Virtual register defs can only be sunk if all their uses are in blocks
// dominated by one of the successors.
// If a previous operand picked a block to sink to, then this operand
// must be sinkable to the same block.
bool LocalUse = false;
- if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, ParentBlock, LocalUse))
- return false;
+ if (!AllUsesDominatedByBlock(Reg, SuccToSinkTo, MBB,
+ BreakPHIEdge, LocalUse))
+ return nullptr;
continue;
}
// Otherwise, we should look at all the successors and decide which one
- // we should sink to.
- for (MachineBasicBlock::succ_iterator SI = ParentBlock->succ_begin(),
- E = ParentBlock->succ_end(); SI != E; ++SI) {
+ // we should sink to. If we have reliable block frequency information
+ // (frequency != 0) available, give successors with smaller frequencies
+ // higher priority, otherwise prioritize smaller loop depths.
+ SmallVector<MachineBasicBlock*, 4> Succs(MBB->succ_begin(),
+ MBB->succ_end());
+
+ // Handle cases where sinking can happen but where the sink point isn't a
+ // successor. For example:
+ //
+ // x = computation
+ // if () {} else {}
+ // use x
+ //
+ const std::vector<MachineDomTreeNode *> &Children =
+ DT->getNode(MBB)->getChildren();
+ for (const auto &DTChild : Children)
+ // DomTree children of MBB that have MBB as immediate dominator are added.
+ if (DTChild->getIDom()->getBlock() == MI->getParent() &&
+ // Skip MBBs already added to the Succs vector above.
+ !MBB->isSuccessor(DTChild->getBlock()))
+ Succs.push_back(DTChild->getBlock());
+
+ // Sort Successors according to their loop depth or block frequency info.
+ std::stable_sort(
+ Succs.begin(), Succs.end(),
+ [this](const MachineBasicBlock *L, const MachineBasicBlock *R) {
+ uint64_t LHSFreq = MBFI ? MBFI->getBlockFreq(L).getFrequency() : 0;
+ uint64_t RHSFreq = MBFI ? MBFI->getBlockFreq(R).getFrequency() : 0;
+ bool HasBlockFreq = LHSFreq != 0 && RHSFreq != 0;
+ return HasBlockFreq ? LHSFreq < RHSFreq
+ : LI->getLoopDepth(L) < LI->getLoopDepth(R);
+ });
+ for (SmallVectorImpl<MachineBasicBlock *>::iterator SI = Succs.begin(),
+ E = Succs.end(); SI != E; ++SI) {
+ MachineBasicBlock *SuccBlock = *SI;
bool LocalUse = false;
- if (AllUsesDominatedByBlock(Reg, *SI, ParentBlock, LocalUse)) {
- SuccToSinkTo = *SI;
+ if (AllUsesDominatedByBlock(Reg, SuccBlock, MBB,
+ BreakPHIEdge, LocalUse)) {
+ SuccToSinkTo = SuccBlock;
break;
}
if (LocalUse)
// Def is used locally, it's never safe to move this def.
- return false;
+ return nullptr;
}
// If we couldn't find a block to sink to, ignore this instruction.
- if (SuccToSinkTo == 0)
- return false;
+ if (!SuccToSinkTo)
+ return nullptr;
+ if (!isProfitableToSinkTo(Reg, MI, MBB, SuccToSinkTo))
+ return nullptr;
}
}
- // If there are no outputs, it must have side-effects.
- if (SuccToSinkTo == 0)
- return false;
+ // It is not possible to sink an instruction into its own block. This can
+ // happen with loops.
+ if (MBB == SuccToSinkTo)
+ return nullptr;
// It's not safe to sink instructions to EH landing pad. Control flow into
// landing pad is implicitly defined.
- if (SuccToSinkTo->isLandingPad())
+ if (SuccToSinkTo && SuccToSinkTo->isLandingPad())
+ return nullptr;
+
+ return SuccToSinkTo;
+}
+
+/// SinkInstruction - Determine whether it is safe to sink the specified machine
+/// instruction out of its current block into a successor.
+bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
+ // Don't sink insert_subreg, subreg_to_reg, reg_sequence. These are meant to
+ // be close to the source to make it easier to coalesce.
+ if (AvoidsSinking(MI, MRI))
return false;
- // It is not possible to sink an instruction into its own block. This can
- // happen with loops.
- if (MI->getParent() == SuccToSinkTo)
+ // Check if it's safe to move the instruction.
+ if (!MI->isSafeToMove(TII, AA, SawStore))
+ return false;
+
+ // FIXME: This should include support for sinking instructions within the
+ // block they are currently in to shorten the live ranges. We often get
+ // instructions sunk into the top of a large block, but it would be better to
+ // also sink them down before their first use in the block. This xform has to
+ // be careful not to *increase* register pressure though, e.g. sinking
+ // "x = y + z" down if it kills y and z would increase the live ranges of y
+ // and z and only shrink the live range of x.
+
+ bool BreakPHIEdge = false;
+ MachineBasicBlock *ParentBlock = MI->getParent();
+ MachineBasicBlock *SuccToSinkTo = FindSuccToSinkTo(MI, ParentBlock, BreakPHIEdge);
+
+ // If there are no outputs, it must have side-effects.
+ if (!SuccToSinkTo)
return false;
+
// If the instruction to move defines a dead physical register which is live
// when leaving the basic block, don't move it because it could turn into a
// "zombie" define of that preg. E.g., EFLAGS. (<rdar://problem/8030636>)
DEBUG(dbgs() << "Sink instr " << *MI << "\tinto block " << *SuccToSinkTo);
- // If the block has multiple predecessors, this would introduce computation on
- // a path that it doesn't already exist. We could split the critical edge,
- // but for now we just punt.
- // FIXME: Split critical edges if not backedges.
+ // If the block has multiple predecessors, this is a critical edge.
+ // Decide if we can sink along it or need to break the edge.
if (SuccToSinkTo->pred_size() > 1) {
// We cannot sink a load across a critical edge - there may be stores in
// other code paths.
if (!TryBreak)
DEBUG(dbgs() << "Sinking along critical edge.\n");
else {
- MachineBasicBlock *NewSucc = SplitCriticalEdge(ParentBlock, SuccToSinkTo);
- if (!NewSucc) {
- DEBUG(dbgs() <<
- " *** PUNTING: Not legal or profitable to break critical edge\n");
- return false;
- } else {
- DEBUG(dbgs() << " *** Splitting critical edge:"
- " BB#" << ParentBlock->getNumber()
- << " -- BB#" << NewSucc->getNumber()
- << " -- BB#" << SuccToSinkTo->getNumber() << '\n');
- SuccToSinkTo = NewSucc;
- ++NumSplit;
- }
+ // Mark this edge as to be split.
+ // If the edge can actually be split, the next iteration of the main loop
+ // will sink MI in the newly created block.
+ bool Status =
+ PostponeSplitCriticalEdge(MI, ParentBlock, SuccToSinkTo, BreakPHIEdge);
+ if (!Status)
+ DEBUG(dbgs() << " *** PUNTING: Not legal or profitable to "
+ "break critical edge\n");
+ // The instruction will not be sunk this time.
+ return false;
}
}
+ if (BreakPHIEdge) {
+ // BreakPHIEdge is true if all the uses are in the successor MBB being
+ // sunken into and they are all PHI nodes. In this case, machine-sink must
+ // break the critical edge first.
+ bool Status = PostponeSplitCriticalEdge(MI, ParentBlock,
+ SuccToSinkTo, BreakPHIEdge);
+ if (!Status)
+ DEBUG(dbgs() << " *** PUNTING: Not legal or profitable to "
+ "break critical edge\n");
+ // The instruction will not be sunk this time.
+ return false;
+ }
+
// Determine where to insert into. Skip phi nodes.
MachineBasicBlock::iterator InsertPos = SuccToSinkTo->begin();
while (InsertPos != SuccToSinkTo->end() && InsertPos->isPHI())
++InsertPos;
+ // collect matching debug values.
+ SmallVector<MachineInstr *, 2> DbgValuesToSink;
+ collectDebugValues(MI, DbgValuesToSink);
+
// Move the instruction.
SuccToSinkTo->splice(InsertPos, ParentBlock, MI,
++MachineBasicBlock::iterator(MI));
+ // Move debug values.
+ for (SmallVectorImpl<MachineInstr *>::iterator DBI = DbgValuesToSink.begin(),
+ DBE = DbgValuesToSink.end(); DBI != DBE; ++DBI) {
+ MachineInstr *DbgMI = *DBI;
+ SuccToSinkTo->splice(InsertPos, ParentBlock, DbgMI,
+ ++MachineBasicBlock::iterator(DbgMI));
+ }
+
// Conservatively, clear any kill flags, since it's possible that they are no
// longer correct.
MI->clearKillInfo();